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Evolution of the Ape Forelimb: Evidence from Internal Bone Structure

Periodic Reporting for period 1 - MOSAIC (Evolution of the Ape Forelimb: Evidence from Internal Bone Structure)

Reporting period: 2016-06-01 to 2018-05-31

Reconstructing how our ancestors interacted with their environment is a primary goal of paleoanthropology as it improves our understanding of human evolutionary history. In particular, researchers are interested in reconstructing the locomotor behaviour of our fossil ape ancestors of the Miocene (20-5 million years ago [Ma]), because it is key to elucidating the origin of a defining feature of being human: bipedalism, or walking on two feet. Unlike today, in which there are numerous species of monkeys but only four groups of great apes (i.e. orang-utans, gorillas, chimpanzees and bonobos), the Miocene is characterised by the opposite pattern, with a relatively greater number of ape taxa. This makes this epoch particularly interesting as there were several apes that may have led to the African ape and human lineage and reconstructing their behaviour can help elucidate what type of locomotion preceded human bipedalism. Thus, this project aims to reconstruct the locomotor behaviour of the Middle-Late Miocene apes, with an emphasis on the evolution of suspensory/climbing and potential knuckle-walking adaptations, to shed new light on the origin of human bipedalism.
The Miocene apes are the direct ancestors of the living apes and exhibit “mosaic morphology”, i.e. they display both primitive (or generalised) and derived (or living ape-like) features that together create combinations of morphologies not found in any living primate, and are characterized by a generalized locomotor repertoire (i.e. arboreal quadrupedalism). There is no consensus among the paleoanthropological community regarding the locomotor behaviour exhibited by the last common ancestor between humans and chimpanzees, and the Miocene apes, with their more generalised morphology, may provide a better ancestral model from which bipedal fossil humans (hominins) – our direct ancestors – evolved.
Thus, the objective of this project is to use the internal structure of multiple forelimb bones to reconstruct the locomotor behaviour of several associated skeletons of Miocene apes, namely: Pierolapithecus catalaunicus (12 Ma, Europe), Hispanopithecus laietanus (9 Ma, Europe), Rudapithecus hungaricus (10 Ma, Europe) and Nacholapithecus kerioi (15 Ma, Africa). The capacity of the trabecular (or cancellous) bone to remodel in response to the loading regime during locomotor behaviours may reflect what the animals were actually doing during their lifetime, rather than what they were capable of doing, thus providing a more direct insight into function and how a bone was used during life.
The specific scientific objectives for the project involved 1) building a comparative sample of modern apes and monkeys; 2) collecting microCT data for the forelimbs of fossil Miocene apes; and, 3) performing holistic analyses of forelimb locomotor signals for living primates and Miocene apes. These objectives were achieved by means of applying state-of-the-art techniques of microCT data analyses in collaboration with the University of Kent (Canterbury, UK) and the Max Planck Institute for Evolutionary Anthropology (Leipzig, Germany) and contextualising the results with biomechanics related to primate locomotion to obtain a holistic interpretation of fossil forelimb morphology and, ultimately, more nuanced reconstructions of their locomotor behaviour. The results indicate that there are subtle but important differences of loading regimes within the different genera of hominoids (living apes) and monkeys albeit their similar use of locomotor behaviour. Gorilla and chimpanzee show different loading patterns (see Figure 1), which could possibly be related to a biomechanically differentiated use of knuckle-walking (columnar-style in gorillas vs extended wrist-style in chimpanzees), which ties in well with the evolutionary hypothesis that knuckle-walking behaviour in gorillas and chimpanzees is the result of a homoplastic process and carries important implications for the evolutionary history of Miocene ape and human locomotion. The results were presented at the 86th American Association of Physical Anthropologists Meeting in New Orleans, 2017, among others.
The project has progressed adequately, achieving most of its objectives and milestones for the period with relatively minor deviations. Expected results include more nuanced and weighted locomotor inferences for Miocene locomotor behaviours, which will shed light on the evolution of suspensory/climbing and potential knuckle-walking adaptations and, ultimately, on the origin of human bipedalism. A better understanding of Miocene ape anatomy and better informed inferences of their locomotion will help clarify the evolution of living hominoids and the origins of their specialised locomotion, including bipedalism. To this regard, many researchers suggest that living apes are far too specialised in their anatomy and locomotion to be plausible models for the Last Common Ancestor between humans and chimpanzees. Miocene apes, on the other hand, may provide a better ancestral model from which bipedal early hominins evolved, owing to their more generalised morphology. Therefore, all evolutionary scenarios for the evolution of locomotion within Hominoidea (including human bipedalism) depend on an accurate reconstruction of locomotor behaviour in Middle-Late Miocene apes.
Figure 1. Chimpanzee flexed elbow-style of knuckle walking vs gorilla extended elbow (columnar)-styl